The containment for a nuclear reactor is defined as the enclosure that provides environmental isolation to the nuclear steam supply system (NSSS) of the plant in which nuclear fission is harnessed to produce pressurized steam. A commercial nuclear reactor is required to be enclosed in a pressure retaining structure which can withstand the temperature and pressure resulting from the most severe accident that can be postulated for the facility. The most severe energy release accidents that can be postulated for a reactor and its containment can generally be of two types.
One thermal event of potential risk to the integrity of the containment is the scenario wherein all heat rejection paths from the plant's nuclear steam supply system (NSSS) are lost, forcing the reactor into a “scram.” A station black-out is such an event. The decay heat generated in the reactor must be removed to protect it from an uncontrolled pressure rise.
Loss-of-Cooling Accident (LOCA) is another type of thermal event condition in which a breach in the pressure containment boundary of reactor coolant system (RCS) leads to a rapid release of flashing water into the containment space. The reactor coolant (primary coolant), suddenly depressurized, would violently flash resulting in a rapid rise of pressure and temperature in the containment space. The in-containment space is rendered into a mixture of air and steam. LOCA events are usually postulated to occur due to a failure in an RCS system pipe containing the primary coolant water. The immediate consequence of a LOCA is rapid depressurization of the RCS and spillage of large quantities of the primary coolant water until the pressure inside the RCS and in the containment reach equilibrium. Nuclear plants are designed to scram immediately in the wake of the RCS depressurization which suppresses the reactor's criticality and stops the chain reaction. However, the large enthalpy of the primary coolant water spilling from the RCS into the containment and the ongoing generation of decay heat in the core are sources of energy that would cause a spike in the containment pressure which, if sufficiently high, may threaten its pressure retention capacity.
More recently, the containment structure has also been called upon by the regulators to withstand the impact from a crashing aircraft. Containment structures have typically been built as massive reinforced concrete domes to withstand the internal pressure from LOCA. Although its thick concrete wall could be capable of withstanding an aircraft impact, it is also unfortunately a good insulator of heat, requiring pumped heat rejection systems (employ heat exchangers and pumps) to reject its unwanted heat to the external environment (to minimize the pressure rise or to remove decay heat). Such heat rejection systems, however, rely on a robust power source (off-site or local diesel generator, for example) to power the pumps. The station black out at Fukushima in the wake of the tsunami is a sobering reminder of the folly of relying on pumps. The above weaknesses in the state-of-the-art call for an improved nuclear reactor containment system.
What is needed is an efficient energy expulsion system to bring the internal pressure in the containment in the wake of a LOCA to normal condition in as short a time as possible. To ensure that such a system would render its intended function without fail, it is further desirable that it be gravity operated (i.e., the system does not rely on an available power source to drive any pumps or motors).